1. Field of the Invention
This invention pertains to the measuring of liquid levels and volumes in tanks and more particularly to techniques for monitoring such levels and volumes in a plurality of tanks, typically mud tanks employed in a drilling operation.
2. Description of the Prior Art
Huge fluid systems, such as mud systems employed in an oil field drilling operation, often need to be closely monitored, for the condition of such fluid system is revealing as to other conditions that exist with the procedure or with the environment of the procedure supported by the fluid system. For example, in a mud system, a high-pressure gas or liquid infusion during the circulation of the mud downhole will be revealed by a rise in the mud reservoir. In similar fashion, drilling into an extremely porous, unpressurized formation will cause fluid loss and a drop in reservoir level.
In addition to monitoring for level changes, it has been common to monitor for mud weight. When initially formulating the "mud" or drilling fluid, such instruments will reveal when the "weight", which equates with specific gravity or density, is proper. Specific gravity is defined as the ratio of the weight of a fixed volume of an unknown liquid divided by the weight of an identical volume of water. Density is a similar term usually applied to solids, but for purposes herein the two terms are used interchangeably. "Mud weight" is also an equivalent term when applied to drilling fluids of liquids. Some instrumentation in the prior art will detect a change of "mud weight" during operation as a sign that something is amiss. For example, encountering a gas pocket during drilling will infuse gas into the fluid and will cause a decrease in the specific gravity of the fluid. Actually, however, in an on-going drilling operation, a readout of "mud weight" is relatively unimportant. A reading of level change is important.
Early instrumentation, and much of the instrumentation used today, for monitoring levels in reservoir tanks utilize floats that ride on the surface of the liquid in the tank. Such floats are usually confined to a wall of the tank so that when the level either rises or falls, the float activates a level sending device mounted on the wall.
Although such actuating devices work well with liquid that is relatively clean and with liquids relatively free from turbulence, floats are notoriously unreliable in conditions that usually exist in a mud reservoir or tank. In fact, any measuring device which is dependent on a rising or lowering movement is susceptible to hang-up. Further, such devices, even when they work, are not extremely sensitive to changes in level. That is, there can be a change of several percent in the level of the tank before a float system will detect this because current float-coupled, level-transmitting devices are of relatively low resolution.
U.S. Pat. No. 4,043,193, John M. Bailey, reveals a non-float system for measuring and calculating the liquid "mud weight" condition of a tank and, therefrom, calculating the tank level. In the system described therein, two independent pressure sensitive elements are employed at two respectively different levels within the tank, each being sensitive to the absolute pressure at its location. The absolute pressure is, in turn, directly dependent on the height or level of the liquid and the specific gravity or density of the liquid. The outputs of the two sensors are applied to a differential pressure transmitter, which produces an output which is a measure of "mud weight" (or, specific gravity or density). The output of one of the two sensors is also applied to an absolute pressure transmitter (which output partially includes the height function). By application of the differential pressure transmitter and the absolute pressure transmitter to a Sorteberg bridge and, by adding a constant that is representative of the distance that the sensor connected to the absolute pressure transmitter is above the bottom of the tank, a measure of the tank depth or level is developed. Volume is then obtained by multiplying the depth by the cross-sectional area of the tank. The electronics required to produce the desirable output in the Bailey system requires, in addition to or instead of, the Sorteberg bridge, non-linear amplifiers, force-balance elements, log/antilog elements, and/or current, voltage or pressure booster devices to achieve the function of multiplication or division for compensating for the liquid density effects on the absolute pressure signal. The use of such active devices to perform the computations required are subject to inaccuracies.
By contrast, applicant's invention pertains to a non-float system for ascertaining tank depth or level without developing the relatively unimportant "mud weight" indication and which does not use critical active devices in such a way so as to inherently introduce errors or inaccuracies in the level readout. The "mud weight" indication is said to be relatively unimportant because changes occurring in specific gravity of the liquid are relatively slow in occurring when compared with changes in level. Therefore, as an alarm indication, changes in level are more meaningful.
Therefore, it is a feature of the present invention to provide an improved level monitor system for a mud or similar tank reservoir which produces a readout without using active non-linear electronic components.
It is another feature of the present invention to provide an improved non-float level monitor system for a reservoir including a plurality of tanks which develops a signal indicative of the total volume within all of the tanks without computing the specific gravity of the liquid within the tanks.